Polarizing plates having a structure in which a protective film is laminated on one or both surfaces of a polarizer formed with a polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resin dyed with dichroic dye or iodine using an adhesive have been commonly used. Triacetyl cellulose (TAC)-based films have been normally used as a polarizing plate protective film in the art, however, such TAC films have a problem of being readily deformed in high temperature and high humidity environments. Accordingly, protective films made of various materials capable of replacing TAC films have been recently developed, and for example, a method of using polyethylene terephthalate (PET), a cycloolefin polymer (COP), and an acryl-based film either alone or as a mixture thereof has been proposed. Among these, an acryl-based film has attracted particular attention with advantages such as excellent optical properties and durability, and being inexpensive.
However, an acryl-based film has higher surface friction compared to films made of other materials, and there are problems in that adhesive strength with an adhesive layer is reduced, workability becomes inferior due to a poor slip property when winding, and a blocking phenomenon, in which film surfaces adhere together, occurs after winding. In order to solve such problems, methods of filling the film with a small amount of rubber particles or inorganic particles when preparing an acryl-based film have been proposed, however, in the case of an acryl-based film prepared using such a method, problems such as the reduction of film transparency due to an increase in haze, and the reduction of film orientation property occur, therefore, an acryl-based film has not been suitable as an optical film.
In addition, a polarizer protective film may generally include various functional coating layers such as an anti-reflection layer and a hard coating layer on one surface opposite to the polarizer-attached surface for anti-reflection, durability improvement, scratch prevention and visibility improvement, and such functional coating layers are generally formed using a method of applying a coating composition including a base resin, a solvent, additives and the like on a protective film, and then curing the result. However, in the case of an acryl-based film, problems such that a functional coating layer is not favorably coated due to poor solvent resistance, or a film surface is melted and damaged when a functional coating layer is formed may occur.
In view of the above, methods of carrying out surface treatment such as plasma treatment and corona treatment on the surface of a protective film, or forming a primer layer have been proposed, however, methods that have been proposed so far have not been able to sufficiently secure adhesive strength between a functional coating layer and a protective film, particularly an acryl-based protective film. For example, urethane-based primers that have been proposed in the art as a primer layer for a protective film have low water resistance and solvent resistance, therefore, there have been problems such that adhesive strength is reduced due to moisture penetration when the primer layer is stored for a long period of time under a high humidity condition, or, when a coating composition including an organic solvent is applied on a primer layer, the primer layer is expanded due to the solvent included in the coating composition or the primer layer is desorbed from a protective film since the primer layer is dissolved in the coating composition.